1. Field of the Invention
The present invention relates to a system and method for controlling an internal combustion engine.
2. Background Art
Internal combustion engines often employ advanced control strategies and after-treatment systems to reduce tailpipe emissions. Conventional closed-loop feedback control strategies have been used to control the fuel/air ratio of the engine to balance engine performance, fuel economy, and emissions. When using an after treatment system that includes currently available exhaust gas catalysts, appropriate control of the engine fuel/air ratio is desired to manage the conversion efficiency of the catalyst for effective exhaust gas treatment. Feedback may be provided by one or more exhaust gas oxygen sensors that provide an indication of the current relative or absolute fuel/air ratio to the engine controller, which adjusts the commanded fuel/air ratio accordingly.
Robust management of catalyst conversion efficiency via fuel/air ratio control is complicated by the catalyst dynamics, which change with air flow rate, catalyst temperature, and catalyst age, for example. The catalyst dynamics are also significantly nonlinear, particularly with a new catalyst. Various engine and vehicle operating conditions, such as idling with a hot catalyst, also present challenges for the engine control system due to the extremely slow dynamics under these conditions.
To effectively control emissions while providing acceptable performance and fuel economy, the present inventors have recognized that the engine controller may incorporate a robust catalyst model that is relatively simple but still represents the catalyst operating dynamics under a broad range of engine and vehicle operating conditions.
To quickly attain a suitable operating temperature for the catalyst and one or more associated exhaust gas oxygen sensors after starting the engine, a small volume catalyst often referred to as a fast light-off or close-coupled catalyst may be positioned near the exhaust manifold. An exhaust gas oxygen sensor is typically positioned downstream of the fast light-off catalyst and is used to monitor the catalyst conversion efficiency. Additional catalysts (or catalyst elements/bricks) are positioned downstream to accommodate packaging considerations and provide a suitable catalyst volume for more demanding operating conditions. However, even though the downstream exhaust gas sensor monitoring the fast light-off portion of the catalyst is appropriately tracking a desired set point, the operating conditions and dynamics of any catalyst (or portion thereof) positioned further downstream cannot be easily determined.